Problem when using Particle world effect with parent camera to null layer?

I started to make a text logo video.
I created 1 text layer and 1 solid layer for particle world and created 1 3D null layer then parent camera layer with it.
After that, I modified some value on particle: gravity, x,y,z position to make particle rain.
The problem is that if I change z position of 3d Null layer, I can see both text layer and Particle layer move following the z axis. But when I set keyframe at 3d null layer's position, just text layer moved to the z axis, the particle layer doesn't change any thing.
Can somebody tell me what is going on? I followed exactly a tutorial from videocopilot.

"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.